This invention relates to a converter system for coupling two high voltage three-phase networks. In particular, this invention is related to a converter system employing a three-phase full-wave converter associated with each of the AC networks to be coupled where the converters include two three-phase secondary windings and a semiconductor subassembly associated with each of the secondary windings.
Known converter systems for providing connections between two asynchronous AC networks include a three-phase full-wave converter arrangement associated with each of the two networks to be coupled.
Each converter arrangement includes two three-phase transformers each with two three-phase secondary windings. One secondary winding of each three-phase transformer is Y-connected while the other secondary winding is delta-connected.
Each converter arrangement further includes a plurality of semiconductor devices which are controlled by a central control circuit. The central control circuit operates such that the converter arrangement connected to the AC network operating as an input network acts as a rectifier. Similarly, the converter arrangement coupled to the AC network which is operating as the receiving network is caused to act as an inverter.
The controlled semiconductors are arranged in the form of uniform functional groups in a modular design. Each functional group includes a valve choke, a number of thyristors, a mounting frame and terminal elements. Each group includes mounting elements for fastening spacer insulators so that a plurality of functional groups may be stacked upon one another to create converter towers or valve towers. The phases of the secondary windings are connected to functional groups in the converter tower.
A decoupled DC circuit in the form of a three-phase to DC to three-phase converter is arranged between two asynchronous networks (networks having different frequency behavior) so as to connect the networks to each other for the purpose of exchanging power. In the three-phase to DC to three-phase configuration, high voltage DC transmission takes place over a very short distance. This type of DC circuit is called a DC short coupler.
A DC short coupler is known from "Oesterreichische Zeitschrift Fuer Elektrizitaetswirtschaft", Vol. 36, No. 8-9 pages 265ff. The arrangement described in this disclosure essentially includes two full-wave converter arrangements which are connected to each other on their DC sides and are connected on their three-phase side to the two networks respectively. The two full-wave converter arrangements contain controlled rectifier components (thyristors) instead of passive rectifier valves. This allows a power exchange between the two coupled AC networks in both directions as desired. The converter arrangement associated with the network that is feeding power is operated as a rectifier and the converter arrangement associated with the network receiving the power is operated as an inverter. The thyristors are controlled via a central electronic thyristor circuit whereby each thyristor is associated with an electronic thyristor circuit assembly which consists of a power unit for forming and delivering an electrical firing pulse, electronic circuitry for processing information and a signal transmission unit. The signals are transmitted in this converter arrangement via light waveguides such as fiber optic cables.
In a further converter arrangement disclosed in DE-OS3404076, two converter transformers are associated with each network. That is, the arrangement includes two primary three-phase windings and two secondary three-phase windings associated with each AC network to be coupled. The secondary windings of the converter transformers are such that one of the secondary windings associated with an AC network is Y-connected and while the other secondary winding associated with the winding is delta-connected. Together the secondary windings act upon a three-phase full wave rectifier having twelve semiconductor rectifier components. Each converter transformer associated with a three-phase network has a secondary winding also having three phases. Each of the three phases for each of the secondary windings is associated with two series connected semiconductor components. Each semiconductor component includes two series connected thyristors. Four such thyristors are included in a semiconductor subassembly. The semiconductor subassemblies are commonly arranged as modules stacked atop each other to form a so called valve tower. As described in this reference, one valve tower is provided for each of the four secondary windings of the converter arrangement. There are two valve towers for each network. According to the disclosure, all three phases of a given secondary winding are coupled to the same valve tower. The reference discloses that the phases are connected to different modules in the same stack or tower of valve modules.
The valve towers are to be mounted in valve sheds or housings in a base area of the converter arrangement. The configuration of providing one valve tower per secondary winding is useful for reducing the base area necessary to contain the converter arrangement. It is sufficient that the valve housing does not require a separate area beneath the towers for control electronics, used to control the semiconductor subassemblies, which are also called base electronics, because relatively short signal transmission paths can be established so long as the valve tower arrangements are not complex. Short signal transmission paths are necessary because control signal transmission is accomplished via light waveguides and the transmission sections which are free of intermediate amplifiers are limited to lengths of about 30 m in fiber optic cables.
This known arrangement fulfills its purpose well for the transmission of relatively low power. For the transmission of higher power, however, this arrangement touches the limit of its economic feasability.
Another approach has been taken to assemble valve towers of the type described above from uniform valve assemblies or modules in a building block fashion. This technique is known from DE-OS3605337.
If the arrangement of DE-OS3404076 is constructed with the modules of the second application and if the transmitted power is to be increased, requiring an increase in the number of modules, a problem arises. In a valve tower associated with a specific secondary winding, a voltage will develop between connection points of the individual phases of the secondary winding and modules of the same tower. The voltage between the individual stories or levels of each tower corresponds to a respective peak value of the AC voltage in question which, of course is higher than the root mean square RMS, voltage by a factor of 2. It is therefore necessary to provide larger insulating spaces between the individual modules when the number of modules and therefore voltage difference are increased.